Up to now section boilers of cast iron or steel plate have been predominantly used for such purposes.
The cast-iron boilers have a high consumption of materials and foundry capacity and because of their high weight are disadvantageous with respect to transport and storage; their repair is complicated. The water paths are of limited cross section and have many direction changes as a result of which, from the viewpoint of circulation, the boiler has a high friction or deflection flow resistance. Because of the high wall thickness, the heat transfer is unsatisfactory and the specific heat transfer coefficient is small. The fabrication cost and thus the cost to the consumer of the cast iron is high. The output of such boilers ranges within narrow limits, e.g. 10 to 20% of the nominal output. Even minor changes in heat requirements necessitate an increase in the number of sections or the size thereof. This is accompanied by a corresponding cost increase in the mounting work.
Steel-plate boilers are constructed in lying or upright configurations. The first have a fire chamber of rectangular or circular cross section and a horizontally arranged burner unit; the latter have a fire chamber of circular cross section and burners mounted at the upper portion.
The partition walls of the boiler in its lying configuration are planar surfaces and either stiffening structures must be used to satisfy strength requirements, or greater wall thicknesses must be selected than may be desirable from the standpoint of the thermal technology. The stiffening gives rise to more complicated fabrication, a higher resistance from the point of view of flow technology, an indeterminate and uncalculatable circulation and, for safety reasons, a reduced specific thermal loading. The higher wall thicknesses, apart from a high cost of materials, result in unsatisfactory heat transfer. The volumetric efficiency of the fire chamber of rectangular cross section is unsatisfactory from the standpoint of the flame. The exhaust gas path has many directional changes and thus a high flow resistance for the exhaust gas. The heat transfer is unsatisfactory because of the planar surfaces, the water scale incrustation on the water side is greater and, as a result, the heat transfer is poor. The dimensions of such boilers are, because of the enumerated disadvantages--with respect to the heat output--fairly large.
With boilers of the upright construction, the boundaries of the radiant and convective heating surfaces create difficulties during the fabrication. For a corresponding state of firing conditions, additional metal-partition walls must be built in with increased labor cost. These walls are subjected to high heat and corrosive effect and should thus be fabricated from better quality materials of greater cost than would otherwise be necessary. The boiler requires costly insulation; its output and its efficiency are low.